Method for sealing a stub tube in a nuclear reactor

ABSTRACT

A method for sealing cracks adjacent stub tube attachment welds in a reactor pressure vessel of a nuclear reactor is described. The reactor pressure vessel includes at least one stub tube through which a control rod housing extends. The control rod housing is attached to one end of the stub tube with an upper stub tube attachment weld. A sidewall of a control rod drive housing is cut at a location below the upper stub tube attachment weld to separate an upper portion of the control rod housing from a lower portion of the control rod housing. The lower portion of the control rod drive housing is removed from the bottom head opening, and a replacement lower portion is positioned in the bottom head opening proximate to the remaining upper portion. The replacement lower portion then is secured to the reactor pressure vessel without securing the replacement lower portion to the remaining upper portion of the control rod drive housing.

FIELD OF THE INVENTION

This invention relates generally to boiling water nuclear reactors and more particularly, to sealing elongate hollow members such as control rod drive housings to a bottom head dome of a nuclear reactor pressure vessel of such reactors.

BACKGROUND OF THE INVENTION

Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick.

The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell.

A plurality of openings are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. Typically, a substantially cylindrical stub tube having a bore extending therethrough is welded to the bottom head dome and the tube bore aligns with an opening in the bottom head dome. The cylindrical stub tube typically is fabricated from a corrosion resistant material such as stainless steel or Ni--Cr--Fe.

As an example, for a control rod drive assembly, the control rod drive housing, e.g., a tube, is inserted through the bottom head dome opening and stub tube bore, and the housing extends into the RPV. The control rod drive (CRD) housing is welded to the stub tube to maintain the housing in the desired position. The stub tube thus serves as a transition piece between the bottom head dome, which typically is fabricated from low alloy steel, and the CRD housing, which typically is fabricated from stainless steel.

Stress corrosion cracking (SCC) is a known phenomenon occurring adjacent to stub tube welds connecting the bottom head dome to the stub tube and connecting the stub tube to the CRD housing. Particularly, the stub tube welds are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Such stresses may, at times, cause cracks adjacent the stub tube welds.

If stress corrosion cracks adjacent stub tube welds are not sealed, such cracks cause potential leakage paths between the stub tube and the bottom head dome, and the stub tube and the CRD housing, respectively, which is undesirable. Accordingly, upon detection of any such cracks, it is desirable to re-seal the control rod drive housing, for example, to the bottom head dome.

One known method of re-sealing CRD housings within the bottom head dome includes completely replacing the stub tube and CRD housing. This method, however, is time consuming, tedious, and expensive. Particularly, the housing and associated stub tube are partially cut-off and the material remaining in the bottom head assembly is inspected to ensure that such material can be welded without damaging the bottom head. A weld build-up is then formed over the remaining material and machined so that a new stub tube can be welded to the weld build-up. Several weeks can be required to perform the replacement process of just one penetration tube. Moreover, much of the replacement work must be performed within the RPV, which requires completely unloading the RPV and is undesirable.

Another known method of re-sealing a CRD housing within the bottom head dome includes welding a sleeve to the CRD housing and the stub. This method, however, only addresses stress corrosion cracks adjacent the interface between the stub tube and the CRD housing. Moreover, installing the sleeve must be performed entirely within the RPV which, as explained above, is undesirable.

Yet another known method of re-sealing a CRD housing within the bottom head dome includes rolling the CRD housing into the bottom head dome. While this method is quicker than replacing the stub tube and CRD housing, rolling the CRD housing into the bottom head dome does not create as tight a seal as a weld between the CRD housing and the dome. Moreover, the rolled CRD housing may become separated from the bottom head dome after continued RPV operation, and must then be re-rolled. Re-rolling a CRD housing, however, often is neither desirable nor practical.

Still another known method of re-sealing a CRD housing within the bottom head dome includes removing and replacing a lower portion of the CRD housing within the bottom head dome. Particularly, a lower portion of the CRD housing is cut-off so that an upper portion of the CRD housing remains inserted in an opening in the bottom head dome and welded to the stub tube. The bottom head dome is then cleaned, and the lower end of the remaining CRD housing upper portion is machined so that a replacement bottom portion of CRD housing can be welded to the remaining upper portion. The replacement bottom portion of CRD housing similarly is machined so that it can be welded to the remaining upper portion. The replacement bottom portion of CRD housing is then inserted into the bottom head dome opening and positioned adjacent the remaining upper portion. The replacement bottom portion and the remaining upper portion of CRD housing are then temper bead welded to each other and to the bottom head dome. Temper bead welding the remaining upper portion to the replacement lower portion and the bottom head dome has the undesirable effect of causing high stresses because of thermal growth mismatch between the CRD housing and the bottom head dome, which are fabricated from different materials. Such temper bead welding also has the undesirable potential effect of trapping water within a leakage path and in contact with the weld between the remaining upper portion and the bottom head dome.

It would be desirable, therefore, to provide a method for sealing a CRD housing within the bottom head dome which can be more easily and quickly performed than known sealing methods. It further would be desirable to provide such a method which reduces stresses caused by thermal growth mismatches between the CRD housing and the bottom head dome.

SUMMARY OF THE INVENTION

These and other objects may be attained by a method which, in one embodiment, facilitates re-sealing a substantially hollow control rod drive housing within a reactor pressure vessel of a nuclear reactor more quickly and easily than known sealing methods. The reactor pressure vessel includes a bottom head having at least one opening extending therethrough, a substantially hollow stub tube, and a control rod drive housing. A lower portion of the stub tube is welded to the bottom head with a lower stub tube weld so that a bore extending through the stub tube is substantially aligned with the bottom head opening. The upper portion of the stub tube is welded to the control rod drive housing with an upper stub tube weld so that the control rod drive housing extends through and is secured within the bottom head opening and the stub tube bore.

In accordance with one embodiment of the present invention, the method facilitates sealing the control rod drive housing to the stub tube if any cracks develop proximate the upper stub tube weld or the lower stub tube weld. The method includes the step of cutting the control rod drive housing at a location below the upper tube weld to separate an upper portion of the control rod drive housing from a lower portion of the control rod drive housing. The lower portion is then removed from the bottom head opening, and a replacement lower portion is inserted in the bottom head opening so that it is substantially adjacent to and aligned with the remaining upper portion. The replacement lower portion is then secured to the bottom head dome without also being secured to the remaining upper portion. Specifically, the replacement lower portion is temper bead welded to a sidewall of the bottom head opening.

The above-described method seals a control rod drive housing within a reactor pressure vessel more easily and quickly than known sealing methods. Such method also reduces stresses caused by thermal growth mismatches between the control rod drive housing and bottom head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a reactor pressure vessel.

FIG. 2 is a partial cross-section view of a control rod drive housing, a stub tube, and a bottom head of a pressure reactor vessel wherein the control rod drive housing is sealed to the stub tube in accordance with one embodiment of the present invention.

FIG. 3 is an enlarged partial cross-section view of the replacement portion and bottom head opening shown in FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a reactor pressure vessel (RPV) 10. RPV 10 includes a top head 12, four substantially cylindrical shell courses 14, 16, 18 and 20, and a bottom head assembly 22. Top head 12 includes a head flange 24. First shell course 14 includes a vessel flange 25. Top head 12 is bolted to first shell course 14 by bolts 26 which extend through head flange 24. Top head 12 also includes a head spray and vent nozzle 28 and lifting flanges 30 used when lifting top head 12 from first shell course 14.

First shell course 14 includes main steam nozzles 32 through which steam flows out of the RPV 10. Stabilizer brackets 34 also are formed on first shell course 14. Second shell course 16 has a number of nozzles 36, 38 and 40 formed therein. Fourth shell course 20 includes a support skirt 42 welded thereto. Support skirt 42 is utilized to support RPV 10 within the reactor housing (not shown).

Bottom head assembly 22 includes a bottom head dome 44 having a plurality of stub tubes 46 welded thereto. Stub tubes 46 are substantially cylindrical and each stub tube 46 has a bore (not shown in FIG. 1) extending therethrough. The bore of each stub tube 46 is aligned with an opening (not shown in FIG. 1) in bottom head dome 44. Components such as control rod drives, incore instruments, pressure instrument nozzles, and drain nozzles extend through such bottom head dome openings and stub tube bores and penetrate into RPV 10.

FIG. 1 is provided primarily for illustrative purposes to show a typical bottom head assembly 22. The present invention, as described below, can be used in many RPV configurations other than RPV 10.

FIG. 2 is a partial cross-section view of a control rod drive housing 50, one stub tube 46, and bottom head dome 44 of RPV 10 wherein control rod drive housing 50 is sealed to stub tube 46 in accordance with one embodiment of the present invention. As shown more clearly, bottom head dome 44 has a thickness TBH, and includes a substantially cylindrical opening 52 therein defined by a sidewall 54. Stub tube 46 includes a first end 56 and a second end 58, and a stub tube bore 60 extends between first and second ends 56 and 58. Stub tube 46 is positioned adjacent bottom head opening 52 so that stub tube bore 60 is substantially aligned with bottom head opening 52. Stub tube 46 is secured to bottom head dome 44 with a lower stub attachment weld 62. Particularly, an outer surface 64 of stub tube sidewall 66 proximate second end 58 is welded to bottom head dome 44 with lower stub tube attachment weld 62.

Control rod drive housing 50 includes a first end (not shown) a second end (not shown) and a bore 68 extending between the first and second ends. Particularly, control rod drive housing 50 has a substantially hollow cylindrical geometric shape including a sidewall 70 having an outer surface 72 and an inner surface 74 which defines bore 68. Control rod drive housing 50 is positioned so that it extends through bottom head opening 52 and stub tube bore 60. An upper portion 76 of control rod drive housing 50 is secured to stub tube 46 with an upper stub tube attachment weld 78 adjacent stub tube first end 56 so that control rod drive housing is substantially concentrically and fixedly secured within stub tube 46. As explained above, the positioning and welding of control rod drive housing 50 within stub tube 46 and bottom head dome 44 is well known. Furthermore, bottom head 44 is fabricated from low alloy steel, and stub tube 46 and control rod housing 50 are fabricated from a compatible corrosion resistant materials such as stainless steel or Ni--Cr--Fe.

Stress corrosion cracks sometimes occur adjacent upper stub tube attachment weld 78 or lower stub tube attachment weld 62. If such a crack occurs adjacent upper weld 78, an annulus leakage path 80 is formed between an inner surface 82 of stub tube sidewall 66 and outer surface 72 of control rod drive housing 50. Similarly, if such a crack occurs adjacent lower weld 62, an annulus leakage path 84 is formed between outer surface 72 of control rod drive housing 50 and sidewall 54 of bottom head opening 52.

To seal annulus leakage paths 80 and 84 in accordance with one embodiment of the present invention, control rod drive housing 50 is cut at a location 86 below upper stub tube attachment weld 78. Particularly, control rod drive housing 50 is cut at location 86 below stub tube second end 58 to separate upper portion 76 of control rod drive housing 50 from a lower portion (not shown) of control rod drive housing 50. The lower portion then is removed from bottom head opening 52 so that only upper portion 76 of control rod drive housing 50 remains inserted in bottom head opening 52.

After removing the lower portion, bottom head opening 52 is cleaned. Particularly, and for example, a grinder (not shown) may be extended into bottom head opening 52 and utilized to grind sidewall 54 of bottom head opening 50 between a lower end 88 of remaining portion 76 and a bottom end 90 of bottom head dome 44. Alternatively, bottom head opening 52 may be cleaned with a flapper wheel or by honing. Methods for cleaning bottom head opening 52 after removing the lower portion of control rod drive housing 50 are known.

A replacement lower portion 92 is then inserted into bottom head opening so that replacement lower portion 92 is substantially aligned with and adjacent to remaining upper portion 76. Replacement lower portion 92 is a substantially hollow member having a substantially cylindrical geometric shape. More particularly, replacement lower portion 92 includes an upper end 94, a lower end (not shown), and a bore 96 extending between upper end 94 and the lower end. Replacement portion 92 is inserted in bottom head opening 52 so that upper end 94 is proximate lower end 88 of remaining upper portion 76 and so that control rod housing bore 68 is substantially aligned with replacement portion bore 96. As shown in FIG. 2, upper end 94 of replacement portion 92 is spaced from lower end 88 of remaining upper portion 76. However, upper end 94 of replacement portion 92 may abut lower end 88 of remaining upper portion 76.

Upper end 94 of replacement portion 92 includes a weld prep 98 for welding upper end 94 to sidewall 54 of bottom head opening 52. Particularly, upper end 94 of replacement portion 92 is machined in accordance with known machining methods to form weld prep 98.

After inserting replacement portion 92 within bottom head opening 52, replacement portion 92 is secured to sidewall 54 of bottom head opening 52 without also being secured to lower end 88 of remaining portion 76. Particularly, upper end 94 of replacement portion 92 is bead welded, using known bead welding methods, to sidewall 54 of bottom head opening 54. More specifically, an automatic welding machine is inserted through replacement portion bore 96 so that a welding head is substantially adjacent to weld prep 98. The automatic welding machine then is used to apply a temper bead weld to sidewall 54 of bottom head opening 52. A UT machine, for example, may then be inserted through replacement portion bore 96 to ascertain the quality of the bead weld.

FIG. 3 is an enlarged partial cross-section view of replacement portion 92 and bottom head opening 52. Upper end 94 of replacement portion 92 is machined so that upper end 94 has a substantially frusto-conical geometric shape. After inserting replacement portion 92 within bottom head opening 52, a temper bead weld 100 is then formed on weld prep 98 to seal upper end 94, and thus replacement portion 92, against sidewall 54 of bottom head opening 52.

To facilitate sealing the control rod drive housing 50 within bottom head dome 44 from underneath bottom head dome 44, a seal (not shown) is installed to prevent water from entering control rod drive housing 50 or leakage paths 80 and 84 during repairs. After completing the repairs, the seal may be removed.

The above-described method facilitates repairing stress corrosion cracks adjacent the upper stub tube weld and the lower stub tube weld more quickly and easily than known methods. In addition, such repairs may be substantially completed from below the bottom head dome, and significantly reduce stresses typically caused by thermal growth mismatches between the CRD housing and the bottom head dome.

From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. For example, while the method was described in reference to a control rod drive housing, such method also may be used to reseal an incore monitor housing or other elongate hollow member within a stub tube and bottom head dome. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims. 

What is claimed is:
 1. A method for sealing an elongate hollow member within a reactor pressure vessel of a nuclear reactor, the reactor pressure vessel including a bottom head dome, a stub tube, and the elongate hollow member, the bottom head dome having at least one opening therein, the stub tube having a first end, a second end, and a bore extending between the first and second ends, the stub tube welded to the bottom head dome adjacent the second end with a lower stub tube attachment weld so that the stub tube bore and the bottom head dome opening are substantially aligned, the elongate hollow member having a first end, a second end, and a bore extending between the first and second ends, the elongate member extending through the stub tube bore and secured to the stub tube adjacent the stub tube first end with an upper stub tube attachment weld, said method comprising the steps of:cutting the elongate member at a location below the upper stub tube weld to separate an upper portion of the elongate member from a lower portion of the elongate member; removing the lower portion of the elongate member from the bottom head opening; inserting a replacement lower portion in the bottom head opening; and securing the replacement lower portion to the bottom head dome without securing the replacement lower portion to the upper portion of the elongate member.
 2. A method in accordance with claim 1 wherein the bottom head opening is defined by a bottom head sidewall, and wherein securing the replacement lower portion to the reactor pressure vessel comprises the step of temper bead welding the replacement lower portion to the bottom head sidewall.
 3. A method in accordance with claim 1 wherein the upper portion of the elongate member includes a lower end, the replacement lower portion includes an upper end, and wherein the replacement lower portion is inserted in the bottom head opening so that the upper end of the replacement portion is spaced from the lower end of the upper portion of the elongate member.
 4. A method in accordance with claim 1 wherein further comprising the step of cleaning the bottom head opening after removing the lower portion of the elongate member from the bottom head opening.
 5. A method in accordance with claim 4 wherein cleaning the bottom head opening comprises the step of honing the bottom head opening.
 6. A method in accordance with claim 4 wherein cleaning the bottom head opening comprises the step of grinding the bottom head opening. 